Petrographic, Mineralogical and Lithogeochemical Study of Core from Three Drillholes into the Steen River Structure, Northern Alberta

The Steen River Structure (SRS) has been the focus of research studies and exploration programs since its discovery in the 1960s. Situated in northwestern Alberta, the SRS lies about 115 km north of High Level along the western border of NTS map area 84N. Lacking a surface expression, the SRS is buried by more than 200 m of Cretaceous to Recent sedimentary units.

The origin of the SRS has been somewhat equivocal. In general, the similarity of its structural geometry, stratigraphy and breccia unit to other known meteorite impact features has supported an extraterrestrial (astrobleme) origin. However, some prior workers have suggested, based on geophysical surveys and magmatic-appearing core from drilling programs, the possibility the structure is, in whole or in part, a volcanic crypto explosion feature.

In early 2000, New Claymore Resources Ltd drilled three diamond-drill holes into the SRS to test three discrete aeromagnetic anomalies that were possibly related to kimberlitic or ultramafic diatreme emplacement. The generalized stratigraphy of the SRS comprises a thin (up to 18.3 m thick) layer of overburden, underlain by undisturbed Cretaceous strata ranging from 224.0 to 275.6 m in thickness, that in turn unconformably overlies a heterolithic, green to reddish-green breccia that is up to 140 m thick Granitic basement underlies the breccia in one of the three drillholes.

Although the SRS breccia in the core visually resembles a kimberlitic diatreme breccia, the results from the detailed mineralogy, petrography and chemical composition obtained in this AGS study indicate that it is primarily a meteorite impact breccia, or suevite. In places, the suevite breccia comprises a melt phase, but this magmatic-appearing rock type is a result of impact shock and heat, and not deep-seated intrusive magmatic activity. In addition, newly formed magnetite was discovered at variable depths within the breccia. The position of the magnetite in the core appears to directly affect the aeromagnetic signature of the SRS.

The SRS is well known as an attractive target for hydrocarbon exploration, and exploitable hydrocarbon resources have been found in several places. However, the economic potential of the SRS to host or have spatially associated metallic-mineral deposits or diamondiferous kimberlites remains uncertain.